Implementing Plant Hydraulics in the Community Land Model, Version 5

Abstract

Version 5 of the Community Land Model (CLM5) introduces the plant hydraulic stress (PHS) configuration of vegetation water use, which is described and compared with the corresponding parameterization from CLM4.5. PHS updates vegetation water stress and root water uptake to better reflect plant hydraulic theory, advancing the physical basis of the model. The new configuration introduces prognostic vegetation water potential, modeled at the root, stem, and leaf levels. Leaf water potential replaces soil potential as the basis for stomatal conductance water stress, and root water potential is used to implement hydraulic root water uptake, replacing a transpiration partitioning function. Point simulations of a tropical forest site (Caxiuanã, Brazil) under ambient conditions and partial precipitation exclusion highlight the differences between PHS and the previous CLM implementation. Model description and simulation results are contextualized with a list of benefits and limitations of the new model formulation, including hypotheses that were not testable in previous versions of the model. Key results include reductions in transpiration and soil moisture biases relative to a control model under both ambient and exclusion conditions, correcting excessive dry season soil moisture stress in the control model. PHS implements hydraulic gradient root water uptake, which allows hydraulic redistribution and compensatory root water uptake and results in PHS utilizing a larger portion of the soil column to buffer shortfalls in precipitation. The new model structure, which bases water stress on leaf water potential, could have significant implications for vegetation-climate feedbacks, including increased sensitivity of photosynthesis to atmospheric vapor pressure deficit.

Big root approximation of site-scale vegtation water uptake

“…Both Ohm's law analogue models revealed their structural limits in representing the important effects of RSA on water potential gradients and thus water uptake. The resistances-in-parallel model overpredicted uptakes in wet layers and excessively redistributed water to dry layers, consistently with performance in LSMs [28]. The resistancesin-series model overestimated uptake in shallow layers and underestimated in at depth.…”

“…This has given an impetus to replace the classical plant water stress formulation with a more process-based model [26,27,28], based on an Ohm's law analogue for water flow in plants [29]. While such an approach works well for stems, its weakness in representing root water uptake is that it places all plant resistances between the root base and each soil layer either exclusively in parallel [29] or in series [30].…”

Dry Deposition of Ozone Over Land: Processes, Measurement, and Modeling

“…To evaluate the strengths and weaknesses of prognostic g s models in simulating stomatal ozone uptake, the community would benefit from better understanding of model sensitivities to parameters and variables as well as their physiological realism. For example, connections between g s and soil moisture and the ability of models to capture such connections (e.g., Anderegg et al, 2017;Bonan et al, 2014;Kennedy et al, 2019;Verhoef & Egea, 2014;S. Zhou et al, 2013) Stomatal ozone uptake changes g s through both short-term and long-term responses.…”

Hydraulics in the 21st century

“…This is not unexpected, as the relevant parameter to be considered is not PLC per se , but the actual hydraulic conductance and its sufficiency to maintain cells hydrated above a critical water content, under a given evaporative demand and residual leaf conductance to water vapor. Models can now simulate hydraulic failure with relatively high accuracy throughout the entire plant vasculature (McDowell et al ., ; Sperry et al ., ), with rigorous hydraulics now entering into ecosystem and global scale models of Earth system processes (Christoffersen et al ., ; Kennedy et al ., ). The identification of thresholds of critical residual hydraulic conductance under different scenarios of evaporative demand suggests models can directly predict mortality from hydraulic failure when they properly represent plant hydraulics.…”

“…Scientific advances in our understanding of plant hydraulics and its implications for plant function have arguably accelerated over the last two decades. New empirical (Holbrook et al ., ; Choat et al ., ) and modeling (Christoffersen et al ., ; Sperry et al ., ; Venturas et al ., ; Kennedy et al ., ; Mencuccini et al ., ) approaches have been applied to tackle some of our largest challenges, and different perspectives have been integrated to better understand the entire vascular system (e.g. carbon metabolism and xylem hydraulics; Hölttä et al ., ; Secchi et al ., ).…”